1. Field of the Invention
This invention is concerned with a current limit control for a pulse-width modulated regulator circuit or a DC-to-DC converter.
Pulse-width modulated converters, especially those using peak current control (PCC) techniques for regulation, experience a condition designated herein as "flare out" at low voltages during current limit overload modes of operation. During flare out, the output current of the converter exceeds its normal controlled upper limit value. Flare out is due to the filtering properties of the PCC current sensing circuitry and to the fixed turn-off delay characteristics of the switching transistors of the converter circuit. Excess current, due to a flare out condition, causes overcurrent stress in the switching transistor. However, while average output current exceeds the limit value, conduction intervals of the switching transistor are too short to allow turn-off loss reduction networks, that may be applied to the switching devices in the converter, to properly reset.
2. Description of the Prior Art
A typical prior art approach in dealing with excess current in an overload mode of operation is the introduction of a foldback characteristic into the current limit function. A typical foldback arrangement for a converter circuit is disclosed in U.S. Pat. No. 3,303,414 issued Feb. 7, 1967 to J. L. Jensen. The output voltage and output current of a converter circuit are simultaneously monitored. At the occurrence of an overload condition, the output current is significantly reduced as the output voltage drops. Hence, the inverter may operate indefinitely under a short circuit or overload condition without causing damage to the current carrying components of the inverter circuit.
Foldback current characteristics are not suitable for high current converter plant applications where extra current is needed to operate a load protection device and quickly isolate a fault without lowering bus voltage. Furthermore, where several high current converters are to be connected in parallel with one another, a foldback protection arrangement lowers the output current too drastically and permits parallel circuits to run indefinitely without operating protection devices.